Lithium batteries have historically employed organic carbonates as electrolyte solvents. However, organic carbonates are typically associated with high degrees of volatility, flammability, toxicity and chemical reactivity. To overcome these disadvantages, electrochemical cells having solid polymer electrolyte (SPE) systems have been prepared in recent years. The ionic conductivity of SPEs was first reported in 1973 for a system of the polyether/alkali metal complex of polyethylene oxide (PEO)/and potassium thiocyanates (KSCN). See D. E. Fenton, J. M. Parker, P. V. Wright, Polymer 14 (1973) 589.
SPE systems have the potential to exhibit numerous advantages over their liquid counterparts, including high energy density, high electrolyte stability, and the ability to be configured in nearly any shape. Such properties are possible because the electrolyte does not contain liquid, has less cost, and is inherently safer. Prior to the 1990's, the research into SPEs had focused on PEO-based electrolytes. The PEO that was typically used was a high molecular weight linear polymer having a semi-crystalline microstructure, and providing relatively strong and free-standing films at room temperature. Such PEO systems may be doped with lithium salt, for example, lithium trifluoromethanesulfonimide (LiTFSI).
Many PEO/lithium salt compositions are predominantly crystalline at room temperature, but melt above about 68° C. As a result, room temperature ionic conductivities are poor (approximately 10−7 S/cm). The improvement in conductivity by using LiTFSI is due to the plasticizing effect of the trifluoromethansulfonimide anion, which substantially reduces the crystallinity of the PEO composition at room temperature. It is important to note that only amorphous PEO electrolytes are ionically conductive. Operation of PEO-based SPEs at elevated temperatures limits their wide application in electrochemical cells.
To increase the room temperature conductivity of PEO, a variety of approaches have been explored. Alkyl phthalates and poly(ethylene glycol)dialkyl ethers with low molecular weights have been used as plasticizing additives for SPE to reduce the crystalline region and increase the mobility of the SPE molecular chain at ambient temperature. Low molecular poly(ethylene glycol)-dialkyl ether compounds can contribute to increased room temperature ionic conductivity of SPEs, but they still exhibit crystallization problems which result in a decrease in the ionic conductivity. See Kelly et. al. J. Power Sources 14, 13 (1985).
Another approach to improving the ionic conductivity at ambient temperature is to synthesize a highly branched PEO to decrease the crystalline tendency of PEO main chain and to increase the chain mobility regarding lithium ion transport such as hyper-branched solid polymer electrolyte (See Z. Wang et. al. J. Electrochem. Soc. 146(6), 2209 (1999); and comb-like solid polymer electrolyte (See R. N. Karekar et. al. Polymer 38(14): 3709 (1997). However, the ionic conductivity is still low even at ambient temperature. All of these attempts were intended to generate an amorphous polymer, at, or near, ambient temperature.
Oligo(ethylene glycol) substituted oligosiloxanes or silanes have also been prepared as ionically conductive polymer hosts. Electrochemical Communications 8, 429-433 (2006). Such materials show conductivities as high as 2 mS/cm at room temperature, when doped with LiTFSI or lithium bis(oxlato)borate (LiBOB). These Si-containing electrolytes have shown excellent performance in electrochemical cells such as lithium ion cells, including long cycle life, low vapor pressure, low flammability and self-distinguish, good electrode wetting ability. It has also been found that the electrochemical working window can be improved (e.g. up to 4.6 V vs. Li+/Li) by adding a silicon-based terminal group to the oligoether chain, i.e. oligo(ethylene glycol) substituted oligosiloxanes/silanes. However, the dimensional stability of these Si-electrolytes is poor and the glutinous nature of the materials flow even under mild pressure at ambient temperature.